1. Field of the Invention
The invention relates to an arrangement for protection against the unauthorized reading of program words stored in a memory notably a program memory, which forms part of a memory device. Said protection arrangement includes a data processor unit having an address output which is connected to an address input of the memory in order to address the stored program words during a processing operation in a given sequence which is determined by the data processor unit.
2. Description of the Prior Art
Devices which are controlled by a data processor unit include a memory, for example, a ROM or a PROM, in which programs and other information (termed software), are stored in the form of program words for the execution of data processing operations. The design of such software is time consuming and is also expensive. Therefore, it is important to prevent unauthorized reading of such software.
A number of methods by which the (software) content of a memory is protected against unauthorized reading are already known. For example, it is known to use code words or access keys, or so-called protection instructions, which prevent access to the program words stored in the memory. Such a method is described, for example, in the article "Computer program protection", by E. J. Sengyel and D. H. Olson; published in I.B.M. TDB Vol. 14, No. 11, April 1972, page 3531.
It is a drawback of the known method that an unsuccessful attempt to copy the software from the memory is noticed rather quickly. An unauthorized reader may then find a way of deactivating the protection method, so that unauthorized reading can still take place.